1. Field of the Invention:
The present invention relates to a high-speed generator direct-coupled to a single-shaft gas turbine engine, etc.
2. Description of the Prior Art:
As illustrated in FIG. 1, a single-shaft gas turbine engine is often designed such that its main turbine shaft 1 is direct-coupled to the shaft of rotor 3 of high-speed generator 2. In such a high-speed generator 2, rotor 3 runs so fast that, when the lubricating oil invades tiny gap 5 between rotor 3 and stator 4, as very large frictional force acts between rotor 3 and stator 4 and the loss due to this force is very large. Moreover, the frictional force imposes an overload on the engine, possibly causing a sudden engine stop. Therefore, it is necessary to avoid invasion of the lubricating oil into tiny gap 5.
Usually, the lubricating oil is stored in an oil tank 6, from which it is pumped up by an oil pump 7 and supplied to various parts of high-speed generator 2, and it is returned to oil tank 6 after lubricating various parts of high-speed generator 2.
As for the oil possibly flowing into tiny gap 5 between rotor 3 and stator 4, there is considered the oil held in stator 4. The oil in stator 4 may flow into tiny gap 5 when the inner cylinder surface of stator 4 is broken. But this invasion of the oil can be prevented by fabricating the inner cylinder surface of, for instance, ceramic which is a heat-resistant material.
As for the lubricating oil for the bearings 8, 9 of rotor 3, there are provided return paths 10, 11, by which the oil which lubricates bearings 8, 9 and which moves toward rotor 3 is returned to oil tank 6, and seals 12, 13, which prevent oil invasion. It would, however, be difficult to completely prevent oil leakage toward rotor 3, no matter what type of seals 12, 13 may be employed, and the slight oil leakage would be unavoidable. This oil leakage is normally so small that the oil which has leaked can be returned to oil tank 6 via return paths 28, 29 without the invasion of the oil into tiny gap 5.
However, as will be explained with reference to FIG. 1, a problem develops when the high-speed generator is direct-coupled to a gas turbine engine.
The annular chamber 15 at the front of the compressor 14 of the gas turbine engine is subjected to negative pressure of, say, about 1500 mmAq on account of the drop in static pressure due to high-speed suction of air and on account of a pressure drop in the air passage from an air cleaner 16 just ahead of the compressor impeller. Annular chamber 15 is cut off by a seal 19 from a cavity 18 formed between air intake 17 and high-speed generator 2. But the seal 19 is not effective enough for complete cutting-off of chamber 15 from cavity 18, and as a result, the air in cavity 18 is sucked into annular chamber 15 on account of a differential pressure between them. Thus the pressure in cavity 18 gradually becomes negative.
On the other hand, the following happens with the pressure in the upper space 20 of oil tank 6. Upper space 20 of oil tank 6 is open to the atmosphere through breather 23 and oil mist separator 24. When the oil is returned to oil tank 6, from bearing 22 between compressor 14 of the engine and turbine wheel 21 of the engine, the air which has leaked into the bearing chamber of bearing 22 flows with the oil into oil tank 6. On account of the air flowing into oil tank 6, a pressure in upper space 20 of oil tank 6 becomes about +100 mmAq. In this state, the air tends to flow from space 20 into cavity 18 formed between air intake 17 and high-speed generator 2. However, return oil path 25 communicating between cavity 18 and space 20 of oil tank 6 is filled up with oil returning from bearings 9, 26. Thus it is difficult for the air to flow in path 25 from space 20 to cavity 18, and the differential pressure between cavity 18 and upper space 20 of oil tank 6 is maintained. Therefore, there also is maintained a differential pressure between cavity 18 and tiny gap 5 which communicates to upper space 20 of oil tank 6 via the path 28. As a result, the air tends to flow from tiny gap 5 into cavity 18, and at the same time the air tends to flow from the bearing 8 into tiny gap 5.
On account of this flow of air, a large volume of the oil which has passed through bearing 8, together with the air, leaks into space 27 through seal 12. A back flow of air also takes place in return path 28 on account of air leaking toward cavity 18. The oil which has gone into space 27 fails to return to oil tank 6, and, instead flows into tiny gap 5 between rotor 3 and stator 4, thereby causing the above-mentioned problem.
Accordingly, although the invasion of bearing oil, used by high-speed generator 2, into the tiny gap 5 at rotor 3 can normally be prevented by sealing, when a gas turbine engine is direct-coupled to high-speed generator 2, a differential pressure develops between the gas turbine side and the opposite side of bearing 8, causing an air flow which is liable to entrain the lubricating oil into tiny gap 5. This problem is particularly exacerbated, when the sealing provided is a noncontact type which is usually desirable with respect to mechanical loss or durability.